My proposed research has not changed since the submission of my 5 year renewal application. It is as follows: (1) Further studies on the organization of capsid polypeptides of model picornaviruses will be pursued. Extracts of cells infected with EMC-virus contain a capsid- related 15S subunit which is thought to represent an intermediate in virus assembly. This subunit will be isolated and characterized by gel electrophoresis, gel filtration and ultra-centrifugation. To further analyze the substructure of this subunit attempts to dissociate it into smaller subunits will be undertaken. Comparative studies on the structure of rhinoviruses 1A, 2 and 14 will be carried out. (2) Studies on the structure of Rous sarcoma and related leukoviruses will be carried out. The aims of this study are: (a) to determine the identity and approximate stoichiometry of each type of polypeptide chain in purified virions; (b) to determine the genetic origin (host cell or virus encoded) of the virion-associated polypeptides, (c) to identify the topographical location and possible function of these polypeptides; (d) to conduct comparative studies on other leukoviruses, especially avian myeloblastosis virus.